In a large saucepan, combine the fava beans, potato, turnip, onion, and 2 cups of the stock. Bring to a low boll over medium heat. Cover and cook until the turnips are tender, 25 to 30 minutes. Allow to cool.

In a blender or food processor, purée the cooked vegetables and their cooking liquid in batches with the remaining 4 cups stock and the 1-1/2 cups cilantro until smooth. Return the mixture to the saucepan. Season with the salt and pepper and heat through. Ladle the soup into individual bowls, garnish with the cilantro leaves, and serve hot.